I use females with my stealthy characters such as in Fallout. I use a silent sniper. (Excluding Saint's Row. I made female characters in that for my serious playthroughs but they're fully clothed. I just really love the voice acting. The men all sound God awful retarded).
I use male characters with my badasses. Like in Mass Effect.

Why? Because I can. There's no real reason beyond that. Hitboxes are usually balanced so they are the same - I hate 3rd person so I always try to be 1st - I don't roleplay.

I usually play male, but I play female in Guild Wars 2 because the males are voiced by Nolan North. I've had enough Nolan North in my ******* lifetime, thanks.

Plus their animations are clunky and most of the armor is rather bland for males. Since I use medium armor, pretty much ALL of the sets are trench coats. At least female sets have a tiny bit of variety, even if they do suffer from the "exposed vitals" shenanigans.

To all the people saying "I'd rather not look at a guy's ass", that's like a tiny portion of the screen, and if it's getting your attention so easily, I would question your confidence in your sexuality.

I think it's funny how all these dudes play as girls because they don't wanna look at a guys ass. If a guys ass distracts you to the point where playing as a female is the only way you can enjoy the game. I have some bad news.

I personally play girl characters in MMOs because I always think like this:
Okay, so I am going to spend hours upon hours of staring at a player model, it could as well be a good looking girl rather than a guy, amirite?